Watery planets beyond the Solar System may be more common than previously thought, making up 35 percent of exoplanets two to four times the size of the Earth. According to a new study, data from the Kepler Space Telescope and the Gaia mission indicate that many planets are made up of half water by mass, as opposed to the 0.02 percent water that the Earth has.

Water is an essential ingredient in the search for extraterrestrial life. It is so basic to any biology as we know it on even the most fundamental level that it is at the very top of the list of exoplanet properties that astrobiologists tick off on their to-do list. Without water, there can be no life, so exoplanet hunters look for planets with positive signs of water or, at the very least, the right conditions were liquid water could exist.

Now an international team led by Li Zeng of Harvard University has found signs that water worlds might be much more common than previously thought. The study found that many of the more than 4,000 confirmed or candidate exoplanets are 1.5 to 2.5 times the radius of the Earth. However, the interesting bit came when they tried to model the internal structure of these Earthlike planets.

"We have looked at how mass relates to radius, and developed a model which might explain the relationship," says Li Zeng. "The model indicates that those exoplanets which have a radius of around x1.5 Earth radius tend to be rocky planets (of typically x5 the mass of the Earth), while those with a radius of x2.5 Earth radius (with a mass around x10 that of the Earth) are probably water worlds"

What this boiled down to is that roughly 35 percent of exoplanets bigger than the Earth are water worlds. That may seem like good news for life hunters, but there's a fly in the cosmic ointment. Far from being planets with vast oceans that might teem with the local equivalent of fish and dolphins snapping up exo-krill, they are actually hellish, with an environment like a pressure cooker.

"This is water, but not as commonly found here on Earth," says Li Zeng. "Their surface temperature is expected to be in the 200 to 500 degree Celsius (392º to 932º F) range. Their surface may be shrouded in a water-vapor-dominated atmosphere, with a liquid water layer underneath. Moving deeper, one would expect to find this water transforms into high-pressure ices before we reaching the solid rocky core. The beauty of the model is that it explains just how composition relates to the known facts about these planets.

"Our data indicate that about 35 percent of all known exoplanets which are bigger than Earth should be water-rich. These water worlds likely formed in similar ways to the giant planet cores (Jupiter, Saturn, Uranus, Neptune), which we find in our own solar system. The newly-launched TESS mission will find many more of them, with the help of ground-based spectroscopic follow-up. The next generation space telescope, the James Webb Space Telescope, will hopefully characterize the atmosphere of some of them. This is an exciting time for those interested in these remote worlds."

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